The United States faces substantial environmental issues from continuing reliance on existing energy sources. The burning of fossil fuels, such as coal and natural gas, results in the emission of excessive amounts of carbon dioxide into the atmosphere. The use of nuclear power raises the specter of ecological damage through the accidental release of radiation into the environment, as well as difficulties in safely disposing of spent nuclear fuel. Hydroelectric projects can disrupt local ecosystems, resulting in major reductions in fish populations, negative impacts on native and migratory birds and damage to river systems.
In recent years, biomass has gained popularity as an environmentally-sound alternative renewable energy source. Biomass, or the fuel products derived from it, can be burned to produce power. Unlike fossil fuels, however, carbon dioxide released from the burning of biomass does not contribute to the overall carbon dioxide content of the atmosphere. This is true because biomass is part of the world's atmospheric carbon cycle. For this reason, biomass is viewed as a renewable, carbon-neutral fuel.
Processing facilities for forest products, used automotive tires, and used railroad cross ties are substantial sources of biomass. The typical forest products facility uses some of its biomass in processing, while the remainder of the biomass is seen as a byproduct. One type of forest products processor that produces a large volume of biomass byproduct is a chip mill that processes small-sized timber. In the chip mill, logs are debarked and then ground into chips for transporting to other mills for further processing. Another type of sawmill is a chip and saw facility (“CNS facility”). A CNS facility produces dimensional lumber from timber that has a diameter ranging from mid-sized to small. Substantial sources of biomass are also available from other forest products facilities, such as large log processing plants, plywood plants, and oriented strand board (OSB) plants, among others.
A typical CNS facility generates an average of more than five-hundred tons of dry biomass byproducts per day. (According to Marks Mechanical Engineering Handbook, the standard for “dry” is defined as twelve percent moisture content or less.) These biomass byproducts typically include white chips, bark, sawdust, and/or wood shavings. The white chips produced by a CNS facility are generally sold to paper-producing mills for processing into paper and cellulose products. The bark, sawdust and shavings are either used at the CNS facility itself as a thermal energy source or sold as a byproduct. Pellet mills have begun to use the white chips and small logs for manufacturing pellets of high density biomass for use a fuel in combustion burner systems. The byproducts of lumber production facilities such as sawdust, planer mill shavings, and bark are not usable for paper production or for pellet production.
Pyrolysis is one process used to produce energy products from biomass. Pyrolysis utilizes temperatures of between about 450-600 degrees Celsius to rapidly heat biomass in the absence of oxygen. The process results in the creation of three products: bio-oil (pyrolysis oil), char, and non-condensing gases. All three products are combustible. Pyrolysis of biomass produces pressure that limits the size and processing capacity of the unit.
Fuel needed to create and maintain such high temperatures in systems utilizing pyrolysis can represent a major operational expense. For this reason, it is desirable to employ systems that make the most of the heat produced. There are a number of strategies for accomplishing this.
One strategy employs techniques meant to optimize the transfer of thermal energy to individual particles of biomass within a pyrolysis chamber. This can be accomplished through the use of organic heat carriers such as hot char and inorganic heat carriers, such as sand. These particularized heat carriers circulate within the pyrolysis chamber and radiate heat to the biomass. Other techniques involve rapidly moving particles of feedstock within a pyrolysis chamber so as to force the particles into nearly continual contact with the hot walls of the chamber. Still other techniques circulate a heated gas stream through a pyrolysis chamber to transfer heat to the particles of biomass. Another strategy involves capturing the hot exhaust resulting from pyrolytic reactions in the pyrolysis chamber and re-circulating that hot exhaust to other parts of the system. Yet another strategy involves insulating the pyrolysis chamber to deter heat loss through the walls of the chamber.
What is needed are pyrolysis systems and methods that improve upon the conservation and re-use of existing heat while being able to produce pyrolysis oil at lower pressures than conventional systems. Also needed are pyrolysis systems and methods that are easily collocated with biomass generating facilities.